Method and apparatus for observing a specimen

ABSTRACT

A method and device for observing a specimen in which an electron beam is irradiated and scanned from an oblique direction, onto a surface of a calibration substrate on which a pattern with a known shape is formed, and an SEM image of the surface of the calibration substrate is obtained. An angle in an oblique direction of the electron beam irradiated is obtained and is adjusted to a desired angle. The electron beam is irradiated from the adjusted desired angle in the oblique direction, onto a specimen substrate on which a pattern is formed, and an SEM image of the specimen substrate is obtained. The SEM image of the specimen substrate is processed by use of the information of the desired angle, and a 3D image of the pattern on the specimen substrate or a shape of a cross section of the pattern is obtained.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/995,388, filed Nov. 24, 2004, now U.S. Pat. No. 7,164,128, thecontents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a method and it's a device used toobserve a specimen using a SEM (Scanning Electron Microscope) in theobservance or measurement of a semiconductor wafer, etc in a process ofsemiconductor manufacture.

It is getting harder to control front-end semiconductor processing as aresult of the increasing miniaturization of semiconductors. Subtlechanges in pattern shapes, such as corner radiuses, as well as theheights, line widths, and sidewall gradient angles of the patterns ofthe semiconductors, make a significant impact on the electricalcharacteristics of semiconductor patterns. Therefore, a technology todetect the changes in a process and control a process by measuring thedimensions or shapes of a semiconductor during manufacture is needed. Atechnology to estimate a 3D profile from the observations of sidewallsor images obtained by an SEM (referred to as an “SEM image” hereinafter)of a semiconductor pattern is expected to effectively control a process.It is supposed that utilizing the information of the SEM images of aspecimen observed in an oblique direction will be effective forobserving the sidewalls or estimating 3D profiles.

The methods to obtain such SEM images observed in an oblique directioninclude, for example, a method to take a tilted image by deflecting anelectron beam that is irradiated from an electron optical system andtilting the direction thereof so as to irradiate the electron beam to anobject of an observation, as described in the Japanese PublishedUnexamined Patent Application (abbreviated to JP-A, hereinafter) No.2000-348658; a method to take an image by tilting the stage itself whichis used to move a semiconductor wafer so that any given position of asemiconductor wafer can be observed by the SEM; and a method to tiltmechanically an electron optical system of the SEM itself.

However, it is expected that the direction of observation (or theincident direction) of an observed image actually obtained usingconventional technologies may have some errors relative to the setvalues. Therefore, the error portion may affect the analysis of theobserved image afterward. For example, an error in the direction of anobservation becomes an issue because dimensional values are changed bythe direction of an observation when detecting a process change bymonitoring dimensions, such as line widths and contact hole diameters(see “Characterization of 193 nm Resist Layers by CD-SEM SidewallImaging”, Proceedings of SPIE Vol. 5038, pp. 892-900, 2003). In the 3Dprofile reconstruction technology used for performing stereo measurementwith images observed from multiple directions, the errors in theobservational directions affect the errors of profiles estimated becauseprofiles are estimated based on the observational directions of multipleSEM images and the disparities among multiple SEM images.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method and apparatusfor observing a specimen. Specifically, the method and apparatus of thepresent invention permit an accurate measurement or observation byaccurately estimating the observational direction of an obliquelyobserved image, calibrating the tilt angle based on the estimatedobservational direction, and using the observational direction as inputinformation for the profile measurement.

In one aspect of the present invention, an electron beam is irradiatedon a specimen having a known shape, electrons discharged from thesurface of the specimen are detected, the incident direction of aconvergent beam is estimated based on the geometric deformation of thespecimen having a known shape by using an image of the intensity of theelectrons detected, and the estimated observational direction is used toanalyze the observed images afterward to allow high-precisionmeasurements. The specimens with a known shape include specimensprepared by using materials having a crystal structure (the angle madeby crystal planes constituting surfaces is known), specimens with asurface profile measured by a measuring means, such as a scanning probemicroscope (SPM), or specimens having a pattern pitch accuracy which isguaranteed by using laser interferometry exposures.

In one aspect of the present invention, the actual observationaldirection can be matched to the set value by adjusting, for example, thepolarization of an electron beam (referred to as “beam tilt”hereinafter), or the tilt angle of a stage or optical system itself(referred to as “stage tilt” or “body tube tilt” hereinafter) based onan estimated observational direction. In the 3D profile estimatingtechnology, the accuracy of estimations can be improved by using theincident direction estimated as an input value of the observationaldirection of an observed image.

In a further aspect of the present invention, an estimated observationaldirection is displayed on GUI (Graphic User Interface) and is referredto when a user analyzes the observed images. A function to convert animage obtained by deflecting an electron beam into an image obtained bytilting a stage and displaying it based on the observational directionis offered. It is difficult to interpret a beam tilt image obtained bydeflecting an electron beam intuitively, because it is geometricallydifferent from an oblique perspective image as seen with the naked eye.Therefore, converting an image into a stage tilt image, which iscomparable to observation by a human obtained by tilting a stage, iseffective for analyses with the naked eye.

In another aspect, to accomplish the object described above, the presentinvention is directed to a method where a convergent electron beam isirradiated and scanned on a specimen, and a specimen is observed by aSEM image obtained by detecting the secondary electrons or reflectedelectrons generated from the specimen by the irradiation; a SEM image ofthe surface of a calibration substrate having a known pattern formedthereon is obtained by irradiating a convergent electron beam from anoblique direction on the calibration substrate and scanning it; an anglein an oblique direction of the electrons being irradiated is obtained byusing information of a pattern with a known shape and the obtained SEMimage; the obtained angle in an oblique direction is adjusted to adesired angle; a SEM image of the specimen substrate is obtained byirradiating an electron beam on the specimen substrate with a patternformed from an oblique direction that is adjusted so that a desiredangle in an oblique direction is obtained, and a 3D image or across-sectional shape of the specimen substrate is obtained byprocessing the SEM image by using information of a desired angle.

In still another aspect, to accomplish the object described above, thepresent invention is directed to a method where a convergent electronbeam is irradiated and scanned on a specimen and the SEM image obtainedby detecting the secondary electrons or reflected electrons generatedfrom the specimen by the irradiation is used to observe the specimen; aSEM image of the surface of a calibration substrate having a knownpattern formed thereon is obtained by irradiating a convergent electronbeam from an oblique direction on the calibration substrate and scanningit; an angle in an oblique direction of an electron beam irradiated byuse of the information of a known pattern shape and the SEM image isobtained; a SEM image of the surface of a calibration substrate having aknown pattern formed thereon is obtained by irradiating a convergentelectron beam from an oblique direction on the calibration substrate; a3D image or a shape of a cross section of a pattern of the specimensubstrate is obtained by processing the SEM image of the specimensubstrate by use of the information of the angle obtained from the SEMimage of a known pattern shape.

According to an aspect of the present invention, the observationaldirection is correctly estimated by using a specimen with a knownpattern formed as a calibrator, thereby allowing more precise andrepeatable measurements of shapes.

According to an aspect of the present invention, an observationaldirection is correctly estimated by using a specimen with a knownpattern formed as a calibrator, thereby allowing the same results evenif the same specimen is observed or measured with different devices.

These and other objects, features and advantages of the invention willbe apparent from the following more particular description of preferredembodiments of the invention, as illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an embodiment of a system according to thepresent invention;

FIG. 2 is a flowchart showing the processing used to estimate a tiltangle;

FIGS. 3( a) and 3(b) are diagrams showing a method to image the amountof a signal of electrons discharged from a semiconductor wafer;

FIGS. 4( a) to 4(d) are diagrams showing a variation of a specimen usinga crystal surface of an embodiment of a calibrator having a known shape;

FIGS. 5( a) to 5(d) are diagrams showing a deformed example of a pyramidspecimen of an embodiment of a calibrator having a known shape;

FIGS. 6( a) to 6(e) are diagrams which show an image of a calibrator,the distribution of the intensity in the observed image of thecalibrator and a method to detect a segment;

FIGS. 7( a) to 7(d) are diagrams which show variations of arrangementsof a calibrator;

FIG. 8 is a diagram showing an example an illustration of straincompensation of an image;

FIGS. 9( a) to 9(d) are diagrams which show an illustration of thepositional relationship among the wafer surface, calibrator and incidentdirection of an electron beam;

FIG. 10 is a schematic diagram of a method used to estimate a tiltangle;

FIG. 11 is a diagram showing an embodiment of an image acquisitionsequence when estimating a tile angle;

FIG. 12 is a diagram showing an embodiment of a procedure to estimate atilt direction of a calibrator by combining image processing, SPM andfocused focal point measurement;

FIGS. 13( a) and 13(b) are flowcharts showing a sequence to image atarget pattern and a calibrator;

FIG. 14 is a flowchart showing a sequence used to save an image of acalibrator, and to refer the information of the database to estimate anincident direction of an electron beam;

FIG. 15 is a flowchart showing a method used to calibrate a tilt angle;

FIG. 16( a) to FIG. 16( d) are partial cross-sections of a pattern toillustrate a principle used to measure a height by a stereovision;

FIGS. 17( a) and 17(b) are diagrams which show an embodiment of a GUI todisplay an estimated result of a tilt angle;

FIG. 18( a) is a perspective view of a pole-shaped specimen;

FIG. 18( b) is a perspective view of a hole-shaped specimen;

FIG. 18( c 1) is a diagram of a pole-shaped specimen observed fromabove;

FIG. 18( c 2) is a diagram of a hole-shaped specimen observed fromabove; and

FIG. 18( d) is a diagrammatic cross-section of a hole 1807 and acalibrator 1808.

FIG. 19 shows a partial cross-section of a pattern.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be explained with reference to FIG. 1 toFIG. 17.

A Method to Observe Tilt with a SEM

FIG. 1 shows an example of a system used to obtain and process a SEMimage. An electron beam source 1303 generates an electron beam 1304. Adeflector device 1306 deflects the electron beam 1304 to control theposition at which the electron beam is irradiated on a specimen, such assemiconductor wafer 1301. The semiconductor wafer 1301 that isirradiated with an electron beam discharges secondary electrons andreflective electrons. The secondary electrons are detected with asecondary electron detector 1309. Meanwhile, reflective electrons aredetected with the reflective electron detectors 1310 and 1311. Thereflective electron detectors 1310 and 1311 are installed in differentdirections from each other the secondary electrons and reflectiveelectrons detected with the secondary electron detector 1309 and thereflective electron detectors 1310 and 1311 are converted into a digitalsignal using A/D converters 1312, 1313 and 1314; and, the digital signalis stored in an image memory 13151 and processed with a CPU 13152depending on the purpose.

FIGS. 3( a) and 3(b) illustrate a method used to image a signal amountof electrons discharged from a semiconductor wafer when scanning anelectron beam on the semiconductor wafer. An electron beam, for example,as shown in FIG. 3( a), is scanned and irradiated in the directions xand y as beams 1501 to 1503 or 1504 to 1506. The scanning direction ofan electron beam can be changed by changing the deflecting direction ofthe electron beam. G₁ to G₃ indicate the respective points of electronbeams 1501 to 1503 that are scanned in the direction x on asemiconductor wafer. In the same manner, G₄ to G₆ indicate therespective points of electron beams 1504 to 1506 that are scanned in thedirection x on a semiconductor wafer. The signal amount of electrondischarged at the points G₁ to G₆ has an intensity value correspondingto pixels H₁ to H₆ in an image 1509 shown in FIG. 3( b) (the subscriptsfrom 1 to 6 attached to G and H correspond to one another). A coordinatesystem 1508 indicates the directions x and y in an image.

In FIG. 1, a computer system 1315 estimates an incident direction of aconvergent electron beam from an observed image of a calibrationspecimen 1319 by performing image processing, estimates a 3D profilefrom an observational image of an object pattern on a semiconductorwafer 1301, or performs a processing and control, such as sending acontrol signal to a stage controller 1320 or a deflection controller1321. A processor and controller 1315 is connected to a display 1316,and it has a GUI (Graphic User Interface) to display an image to a user.An XY stage 1317 moves the semiconductor wafer 1301, thereby allowingthe semiconductor wafer to be imaged at any given position. FIG. 1 showsan embodiment with two detectors of a reflective electron image. It ispossible to increase or decrease the number of detectors of thereflective electron image.

The methods for a SEM to observe an object that is tilted include (1) amethod to take a tilted image by deflecting an electron beam irradiatedfrom an electron optical system and tilting the direction to irradiatethe electron beam, as disclosed in JP-A-2000-348658 (referred to as a“beam tilt method” hereinafter, and the image obtained is called a “beamtilt image”); (2) a method to tilt the stage 317 itself that moves thesemiconductor wafer (in FIG. 1, the stage is tilted at a tilt angle1318) (referred to as a “stage tilt method” hereinafter, and the imageobtained is called a “stage tilt image”); and (3) a method tomechanically tilt the electron optical system itself (referred to as a“body tube tilt method”, and the image obtained is called a “body tubetilt image”. It is required to know the correct incident direction of aconvergent electron beam in order to detect a process change and controla process by measuring a line width or a contact hole diameter, or forperforming a precise analysis, such as an estimation of a 3D profileprecisely from a SEM image obtained by the method. However, the currenttechnology can not determine how much an actual incident direction isdeviated from a set value of an incident direction of a convergentelectron beam. Therefore, a method to correctly estimate the actualincident direction is needed.

A Basic Idea of the Present Invention

In order to solve the above-stated problem, the present invention hasadopted a method to estimate an incident direction of a convergentelectron beam by irradiating a convergent electron beam on a specimenwith a known shape, detecting electrons discharged from the surface ofthe specimen, using an image of the intensity of the electrons detected,based on a geometric deformation of the specimen with a known shape onthe image, to determine the incident direction. In other words, anincident direction (a tilt angle) of an electron beam is estimated onthe basis of an orientation of the specimen with a known shape (acalibrator). If the orientation of a calibrator relative to an absolutecoordinate system of an electron optical system can be measured, anincident direction of an electron beam to an absolute coordinate systemof the electron optical system can be measured.

An absolute coordinate system of an electron optical system is acoordinate system having a z-axis oriented at an incident direction (acentral axis of an optical system) of an electron beam at a beam tiltangle 0°, an x-axis orthogonal to the z-axis on a plane surfaceincluding electron beams 1501 to 1503 that are scanned in the xdirection shown in FIG. 3( a), and, likewise, a y-axis oriented in adirection orthogonal to the z-axis on a plane surface including electronbeams 1504 to 1506 that are scanned in the y direction.

If the orientation of a calibrator to an absolute coordinate system ofan electron optical system can not be measured, an incident direction(it may be slightly different from a set value) of an electron beam to acoordinate system specified by some criteria (for example, a coordinatesystem with a z-axis oriented in an incident direction of an electronbeam when a set value of a beam tilt angle is 0°) can be measured.

FIG. 2 shows a sequence used to estimate a tilt angle in accordance withthe present invention. First, a beam tilt angle, a stage tilt angle or abody tube tilt angle of a device is set in a step 1401 (a beam tiltangle, a stage tilt angle and a body tube tilt angle are collectivelynamed “a tilt angle” hereinafter). A calibrator is observed in a step1402. The calibrators used here include (1) a specimen utilizing acrystal plane (an angle made by crystal planes constituting a surface isknown), (2) a specimen with a surface profile measured by a measuringmeans, such as a scanning probe microscope (SPM) or (3) a specimen witha pattern pitch accuracy guaranteed by using laser interferometryexposure.

In a step 1403, a tilt angle is estimated based on a geometricdeformation of a calibrator on an observed image and a deviation of anincident direction of convergent electrons from a set value in the step1401 is detected. The step 1403 is composed of the steps 1404 and 1405.A section where the tilt of the shape of a specimen changessignificantly shows a difference in the appearance of a specificcalibrator (a ridge or valley). This is called an “edge” hereinafter.

In a step 1404, an edge is detected in an observational image of acalibrator. In a step 1405, a tilt angle is estimated based on thegeometric deformation on an observational image of the calibrator. Atthis moment, all or a part of the information concerning the shape,orientation and position of the calibrator 1406, the orientation andposition of the wafer surface 1407 or the image distortion 1408 arerequired for estimating the tilt angle. Therefore, in the step 1405, asthe tilt angle, a part or all of the information 1406 to 1408 isestimated based on the geometric deformation on an observational imageof a calibrator, or a part or all of the information 1406 to 1408 isestimated by combining the distance measuring means using imageprocessing, SPM or a focused focal point position on an objective lens,etc. In order to estimate a tilt angle, at least, the orientation of acalibrator should be known or estimated among the information 1406 to1408.

An estimated tilt angle is used for (1) calibrating the tilt angle(matching a set value of a tilt angle to an actual tilt angle, (2)effecting improvement by using an estimated tilt angle as an input valueto a stereo measurement, or (3) observing or measuring an observationalimage by displaying the tilt angle on a GUI (Graphic User Interface) fora reference of an observation and analysis of a user. The details ofeach step will now be explained.

A Calibrator with a Known Shape

Types of Calibrator

The specimens with a known shape that is used as a calibrator include,as mentioned above, (1) a specimen utilizing a crystal plane (an anglemade by crystal planes constituting a surface is known), (2) a specimenwith a surface profile measured by a measuring means such as a scanningprobe microscope (SPM), (3) a specimen with a dimensional accuracyguaranteed by a pitch formation using laser interferometry exposure, or(4) a specimen that is usable for a standard.

The specimens of the type (1) will now be explained. Crystal anisotropyetching is a method used to constitute a specimen surface with a crystalplane. That is an etching technology using a property in which theetching rate differs from crystal plane to crystal plane. For example,in an Si crystal, the etching rate for a Miller's index plane (111) ismuch lower than the etching rate of a plane (100) or (110). Therefore,etching proceeds in such a manner that the plane 111 with a low etchingrate comes to the top.

FIGS. 4( a) to 4(d) show examples of specimens constituted of Si crystalplanes. A specimen with a Quadrangular pyramidal concave portion 101(referred to as a “concave pyramidal type specimen” hereinafter), asshown in FIG. 4( a), is composed of a plane (111) and a planes (111)(111) (111) that have the direction of a plane equivalent to the plane(111). Therefore, a tilt angle between crystal planes is known. FIGS. 4(b), 4(c) and 4(d) show a specimen with a Quadrangular pyramidal convexportion 102 (referred to as “concave pyramidal type specimen”), an anglespecimen 107 (tilt angle tan-1(√2)°) and a line and space type specimen108 (tilt angle 90°), respectively. Other than them, specimens usingvaried crystal, planes of not only Si crystals, but also GaAs crystalscan be generated.

Variations of the pyramidal specimens include a pyramid sample with anupper portion (corresponding to the area around the vertex P₀ in FIG. 4(b)) of the pyramid being flat and a plane formed by vertexes Q₅ to Q₈,as shown in FIG. 5( a), a specimen that looks like a deformed pyramidwith a round upper portion of a pyramid, or a deformed pyramidalspecimen with an area (the area around Q₀-Q₁, Q₀-Q₂, Q₀-Q₃, Q₀-Q₄ inFIG. 4( b) on which planes constituting a pyramid are ground to be flat,as shown in FIG. 5( c).

The specimens of the type (2) will now be explained. Distance data isobtained by measuring a shape of a surface of a specimen with ameasuring means, such as a SPM. The angles made by planes constitutingthe surfaces of the specimen are obtained through a polyhedralapproximation by performing local plane applications. Any givenspecimens which are appropriate for observing a SEM image by themeasurement method can be used.

The specimen of the type (3) will now be explained. The pitch of thepitch pattern is known because the exposure using an interference of alaser forms a pitch pattern with a high degree of accuracy.

The specimens of type (4) will now be explained. For example, specimensof the pole and hole types shown in FIG. 18( a) and FIG. 18( b),respectively, can be used for a standard in the observational direction(the shapes where a section is getting larger or smaller towards thecenter axis direction of a pole or hole as shown in the drawings areincluded). FIG. 18( c 1) and FIG. 18( c 2) are examples of observationalimages of FIG. 18( a) and FIG. 18( b) observed approximately from above.For example, when the hole of FIG. 18( b) is opened perpendicular to awafer upper surface 1809, if the center locations of the upper surface1804 and the lower surface 1803 of the hole are matched to each other inan observed image, as shown in FIG. 18( c 1), the observationaldirection is judged to be perpendicular to a wafer. In other words, itcan be used as a standard in the observational direction. Anobservational direction can be judged vertically with a higherresolution if a hole is deep because the center location s of the uppersurface 1804 and the lower surface 1803 of the hole are deviatedrelative to each other, as shown in FIG. 18( c 2), even when theobservational direction is slightly deviated from the verticaldirection.

Specimens including a sharp tilt typified by the pole or hole type areadvantageous because the specimen can be confined in a visual field, theresolution is high and the range of changes in height in the z-directionof a calibrator is large. In addition, the deviation from the verticaldirection of an observational direction can be estimated by a geometricrelationship with a deviation of the center location. Further, anobservational direction can be calibrated in the vertical direction byincluding the center location. Further again, if a hole is bored in anygiven direction relative to the upper surface of a wafer, it is possibleto judge whether the observational direction is the given direction, toestimate the amount of deviation of an observational direction and thegiven direction or to calibrate an observational direction relative tothe given direction.

If the direction of a hole bored is once cleared by a measurement, etc,after that, it can be used as a standard for the direction of the holebored. Also, in the pole type specimens, as a specimen of the abovementioned hole type, it is possible to judge, estimate or calibrate anobservational direction.

In addition, an observational direction that can be judged, estimated orcalibrated with any given standard typified by the pole or hole typealso can be measured with any given calibrator. In other words, as anexample shown in FIG. 18( d), in an observational direction where thecenter location of the upper surface 1805 of a hole 1807 is aligned tothat of the lower surface 1806 of the same, as shown in FIG. 18( c 1),for example, the direction of the tilt (an observational direction thatcan be judged, estimated or calibrated with a standard) of the hole 1807can be obtained by observing the pyramidal calibrator 1808 andestimating the observational direction.

With regard to the definition of beam a tilt angle of 0°, the z-axisdirection in an absolute coordinate system where the central axis of theoptical system is the z-axis may be set to 0°. Or, any given verticaldirection in appearance relative to a wafer surface or any givenobservational direction defined with a standard, etc also may be set to0°.

An observational direction is estimated based on the differences in theappearances of the specimens by considering differences in theobservational directions. For estimations of observational directionsdescribed below, something characteristically indicates a geometricalshape of a specimen on an observed image, such as the length or tiltangle of the segment 301 to 308 of the concave pyramid shown in FIG. 6(a), or a part or all of the coordinate value of the vertexes P₀ to P₄shown in FIG. 6( a) should be detected and utilized (the vertexes P₀-P₄correspond to R1-R6 in FIGS. 4( a)-4(d) and FIGS. 6( a)-6(e)). Thesegments correspond to the “edge” where the slope of the shape of aspecimen changes significantly. They are segments 309 to 311, etc in aspecimen of the line and space type as seen in FIG. 6( b).

A method to detect an edge from an observed image through imageprocessing will now be described. For an observed image, either asecondary electron image taken from a signal amount detected with thesecondary electron detector 1309 or a reflective electron image takenfrom a signal amount detected with the reflective electron detectors1310 and 1311 is used.

In a secondary electron image, a difference in the slope of the shape ofa specimen is reflected in a difference in the signal amount. Inaddition, a phenomenon called “edge effect” is observed at an edge partwhere the slope of the shape of a specimen changes significantly. Alarge signal amount is detected in an angular shape. On the contrary, asmall signal amount is detected in a volley shape. Thus, an edge can bedetected by detecting a part with a signal amount changing or a part ofa peak with a large or small signal amount.

Also, in an image of a reflection electron, a difference in the slope ofthe shape of a specimen is reflected in a difference in the signalamount. Therefore, an edge can be detected by detecting an area withsignals that are changing on an image. In a reflective electron image,the edge effect does not occur. Therefore, in some cases, it isdesirable to use an image of a reflective electron to improve theaccuracy of detecting an edge if an edge is observed that is muchexpanded due to the edge effect.

Each segment of a calibrator can be detected by detecting several areaswhere an edge with the slope of the shape of the specimen issignificantly changed on a segment to be detected or there around and byapplying a straight line to them. The detection of a segment 304 in thepyramidal specimen shown in FIG. 6( a) will now be explained by withreference to FIG. 6( a 2), where an area 324 enclosed with the dottedlines in FIG. 6( a) is enlarged. Several edges are detected on thesegment 304 or the around in FIG. 6( a 2) corresponding to the area 314in FIG. 6( c).

Black dots 321 indicate positions of multiple edges detected at anygiven interval along the segment 304. In some cases, the multiplepositions where an edge is detected deviate and do not exist on a linedue to the noises of images, etc. However, a straight line (i.e.,segment 304) can be accurately detected by applying a straight line tothe multiple positions 321 of edges by means of the least-square method,etc. to reduce the effect of the noise. However, if a large exceptionalvalue with a propensity that is different from other points is includedin the edge position 321 (for example, an edge position 322 spaced farapart from the straight line), a processing not considering anexceptional edge position can be selected by excluding an exceptionaledge position that is spaced far apart from a detection line, or byperforming a least-square method with each edge position weighted so asto be inversely proportional to the distance from a detection line ofthe edge position because a value estimated by the least-square methodpossesses lower reliability.

For example, when detecting the edge position 321, the approximateposition of the segment 304 known in advance is convenient for use infinding an approximate detection range or detection direction, etc.Therefore, the approximate position of the segment 304 is determinedbefore detecting the edge position 321 by a detection processing whichis described below by way of example. At first, an entire image isdifferentially filtered to extract an approximate probable position ofan edge from the image. An approximate position of each segment isestimated among points obtained by binarizing the output values of adifferential filter by means of the Hough transform, which is commonlyknown as a voting-type line detection method.

Some geometric shapes of calibrators include a circular pattern, asshown in FIGS. 18( a) and 18(b). It is possible to express a geometricalshape quantitatively by applying a circular or curved pattern to an edgedetected in the same manner as a straight line, or directly using acoordinate of an edge.

Advantages of Using Pyramid Specimens

A pyramidal (quadrangular pyramidal shape) has the following advantageswhen detecting these segments in a SEM image by image processing.

(1) Both sides of the edge of the specimen are symmetrical Therefore,the distribution of the signal amount in an observed image is alsosymmetrical. Therefore, it is possible to detect an edge in an observedimage with a high degree of accuracy.

(2) It is possible to estimate a two-dimensional (x and y direction)observational direction by observing a pyramid.

With regard to item (1) mentioned above, FIG. 6( c) shows thedistribution 312 of the amount of the SEM signals at the segment A-B ina concave pyramid and the distribution 313 of the shape of the surfaceof a specimen. Both sides of the position 314 of the straight line 304to be detected are substantially symmetrical. Therefore, thedistributions of the amount of the signals are also symmetrical. Becausethe deviation of the signal amount to one side is small, the position ofa straight line that is close to the true value can be detected. Thesame propensity is observed in an angle specimen where either side of astraight line to be detected on the surface of the specimen issymmetrical, as shown in FIG. 4( c). This propensity is notsignificantly lost if the observational direction changes only slightly.

Meanwhile, FIG. 6( e) shows the distribution 318 of the SEM signalamounts at the straight line E-F and the distribution 319 of the shapeof the surface of a line and space type specimen. Because the shape oneither side of the position 320 on the straight line 310 to be detectedis different, the distribution of the signal amount on either side isalso different resulting in a propensity of the signal amount to deviateon either side. Therefore, the detected position is in danger ofdeviating from a true value. However, if the signal amount deviateslargely, it is possible to correct the deviation of the signal amountand improve the detection accuracy by estimating the distribution ofgenerated signals expected from the shape of a specimen by the MonteCarlo simulation, etc.

If an observational direction is estimated by detecting the coordinatevalue of the vertexes P₀ to P₄ shown in FIG. 6( a), for example, anvertex may be obtained from an intersecting point of the straight linesdetected. For example, the vertex P₀ and the vertex P₁ can be obtainedfrom an intersecting point by a combination of a part or all of thestraight lines 301 to 304, and a combination of a part or all of thestraight lines 301, 305 and 306, respectively. FIG. 6( a 1) shows anexample of the vertex P₀ detected, where the area 323 enclosed by adotted line in FIG. 6( a) is enlarged. When estimating a vertex fromthree or more straight lines in FIG. 6( a 1), in some cases, theintersecting points P_(0a) to P_(0d) detected respectively from anintersecting point of the two straight lines arbitrarily selected maynot meet one another. In this case, the deviation in detecting straightlines can be reduced by using the centroid or median point of themultiple intersecting point for the vertex P₀.

FIG. 5( b) shows the specimen of FIG. 5( a) when observed in a directionperpendicular to the wafer surface. With regard to a deformed specimenin a pyramidal shape having a flat upper portion, the geometricdeformation of a pyramid on an observed image can be estimated bydetecting the vertexes Q₅ to Q₈ or the segments 1605 to 1608 whoseequivalents do not exist in the convex pyramid in FIG. 4( b). Meanwhile,the geometric form of this deformation also can be expressed by thevertexes or segments of normal pyramidal specimens. In other words, ageometric deformation of a pyramid can be estimated by extending thesegments 1601 to 1604, respectively, and by detecting a virtual vertexQ′₀ corresponding to the vertex P₀ in FIG. 6( a) from the intersectingpoint. (The vertex Q′₀ can be detected by the same center-of-masscalculation as that of FIG. 6( a 1)). It is possible to perform ananalysis which is the same as that for the normal pyramid shown in FIG.6( a) by using the virtual vertex Q′₀.

As shown in FIG. 5( c), in a specimen where an edge on which two planesconstituting a pyramid meet each other is cut off so as to provide flatsurface, there is a method to use a segment that does not exist in anormal pyramidal specimen as a characteristic of the shape. For example,a virtual vertex Q′₁ corresponding to the vertex P₁ in a normalpyramidal specimen as seen in FIG. 6( a) can be detected by extendingthe segments 1613 to 1614, respectively, and calculating an intersectingpoint. Further, the virtual straight line 1609 corresponding to thestraight line 301 in FIG. 6( a) can be detected by extending thesegments 1617 to 1618, detecting the virtual vertex Q′₅ (correspondingto the vertex Q₅ in FIG. 6( a)), and producing a line from the virtualvertex Q′₁ to Q′₅. Further, the virtual vertex Q″₀ corresponding to thevertex P₀ in FIG. 6( a) can be detected from an intersecting point madeby a combination of a part or all of the virtual straight lines 1610 to1612 detected in the same way as the virtual straight line 1609. It ispossible to perform an analysis which is the same as that for the normalpyramids shown in FIG. 6( a) by using the virtual straight lines 1609 to1612 or virtual vertex Q″₀.

With regard to the item (2) mentioned above, it is possible to estimatea two-dimensional observational direction by observing a pyramid becausethe slopes in the x- and y-observational direction of a pyramidalspecimen are both expressed in accordance with the displacement of thevertex of the pyramid. Meanwhile, in the angle specimen and the line andspace type specimen shown in FIGS. 4( c) and 4(d), respectively, thechanges in the observational direction without an edge (e.g., thedirection from the vertexes R₁ to R₂ in FIG. 4( d)) can not beestimated. However, it is possible to estimate a two-dimensionalobservational direction by disposing multiple specimens in such a mannerthat the directions of the edges are different from each other (e.g.,the example of disposition 709).

Size and Disposition of Specimens

The size and disposition of specimens will now be explained withreference to an example of a pyramidal specimen.

The size of a specimen should be so determined that (1) it is containedin the visual field when imaging, (2) it can observe a sufficient lengthof segment to secure a detection accuracy of each segment of a pyramid,and (3) it has a sufficient height to raise the resolution of thedisplacement of a pyramid on an image detected when the observationaldirection changes. The appropriate size of a pyramid is about 0.1micrometer to a few micrometers in the case, for example, when theimaging magnification and focal depth are 30 k to 150 k and a fewmicrometers, respectively, depending on the imaging magnification andfocal depth of the SEM.

The location and method which are employed to dispose a specimen willnow be explained. The positions 704 and 705 in FIG. 7( a) are thecandidate sites to install a specimen. The specimen is attached toposition 704 on the semiconductor wafer 702 (corresponding to 1301 inFIG. 1), which is installed on the stage 701 (corresponding to 1317 inFIG. 1), or it is attached to position 705 on the holder 703, which isinstalled on the stage 701. In order to attach a specimen on position704, it is required to prepare a wafer that is dedicated for acalibration where a calibrator is formed instead of the semiconductorwafer 702 and to attach it instead of the semiconductor wafer 702 or toattach the semiconductor wafer 702 that is provided with a calibrationpattern when forming a semiconductor pattern.

In general, it is difficult to generate a calibration pattern, such as apyramid, simultaneously with the semiconductor pattern. However, it isalso possible to measure the semiconductor pattern generated on thesemiconductor wafer 702 with a measuring means, such as a SPM, and touse it as a calibrator with a known shape. In addition, as shown in 704,the distortion of the stage or distribution of changes in theobservational direction inside the plane caused by changes in theelectric field due to the movements of the stage can be measured bydisposing several specimens (nine specimens are used in position 704)leaving a space among them. Examples for disposing calibrators atposition 704 or 705 will now be explained with reference to FIG. 7( b).

In the example 706 of the disposition shown in FIG. 7( b), pyramids 710(corresponding to 101 or 102 in FIG. 4) of a predetermined size aredisposed at a regular interval. Disposing multiple pyramids has theadvantages of being able to (1) estimate the observational direction ofeach pyramid, use the average or median of the observational directionand reduce the deviation of the estimated values in the observationaldirection caused by the individual differences and image noises, (2)observe multiple pyramids in a single visual field and measure thedistribution inside the visual field in the observational direction, and(3) observe other specimens when some specimens are contaminated andkeep on favorable calibrations.

Further, as shown in FIG. 7( c), a distortion of an image in a visualfield can be estimated and corrected by observing multiple pyramids witha known disposition (disposed in a reticular pattern in FIG. 7( c)) in asingle visual field. In other words, (1) the scanning direction(corresponding to the x- and y-direction in an image) of a convergentelectron beam is changed, or (2) a distortion of an image is correctedby re-configuring an image without any distortion by geometricallydeforming an obtained image so that the line A-B originally forming astraight line connecting the vertexes of the pyramids in FIG. 7( c) isconverted in the straight line C-D. A distortion of an image inside avisual field can be estimated and corrected by observing a singlepyramid. In other words, for example, if the segments 301 to 308 of theconcave pyramid in FIG. 6( a) are guaranteed to be straight, and thesegments 301 to 308 are observed to be curved (e.g., an observed imageof a concave pyramid in FIG. 7( d)), it is possible to adopt adistortion correcting means to correct the curved line to a straightline. If this kind of observed image is distorted significantly, it isnecessary to correct the distortion of the observed image in order toestimate a tilt angle with a high degree of accuracy.

A method has been proposed to correct a distortion between the beam tiltimages with different tilt angles by matching the bottom shapes ofpyramids among images. In other words, focusing attention on a propertythat is specific to the beam tilt images in which the shape of thegeometric pattern on a horizontal plane is stable against the x- andy-plane of an electron optical system irrespective of the beam tiltangle in the beam tilt images, for example, if a wafer surface ishorizontal to the x- and y-plane of an electron optical system, thebottom shape of a pyramid on a wafer surface is stable irrespective ofthe tilt angle. Therefore, it is assumed that the difference in theshape among the beam tilt images is caused by a distortion in imaging.Consequently, the information about parameters used to express theimages, segments, vertexes or shapes obtained without any distortions isobtained by correcting the parameters (e.g., gradient angle of thesegments) expressing the images, or segments, vertexes or shapes of apyramid detected in such a manner that the bottom shapes of pyramidsmatch among images. In other words, it is possible to obtain an observedimage with less distortion and estimate an observational direction witha high degree of accuracy.

FIG. 8 shows an example of the distortion correction processing used tomatch the bottom surface shape of a beam tilt image to the bottomsurface shape of the top-down image on the basis of the bottom surfaceshape of a top-down image. The bottom surface shapes usable for astandard include a bottom surface shape obtained from a top-down image,a bottom surface shape obtained from a beam tilt image observed by anygiven beam tilt angles, or an average bottom surface shape obtained frommultiple images in the same or different observational directions. Inmany cases, a top-down image has less image distortion than a beam tiltimage. Therefore, a bottom surface shape obtained from a top-down imageor an average bottom surface shape obtained from multiple top-downimages is often effective.

The bottom shape 1701 in FIG. 8 (lines sequentially connecting vertexesP₁ to P₄) is a bottom surface shape in the top-down image 1704. Adeformation parameter to correct the bottom shape 1702 in beam tiltimage 1705 to match the shape 1701 is obtained. A beam tilt image 1706,after distortion correction, is obtained by correcting a distortion ofthe beam tilt image 1702 with the deformation parameter. The bottomshape 1703 in the beam tilt image 1706 after distortion correction ismore similar to the bottom shape 1701 than the bottom shape 1702.

In the disposition example 707, as seen in FIG. 7( b), the pyramids 710having the same size are arranged without intervals among pyramids. Theadvantages in such an arrangement include a measurement allowed in thesame manner as the disposition example 706, and substantially the samedistribution of the SEM signal amount of the bottom of a pyramid(segments 305 to 308 in FIG. 6( a)) at the both sides of a bottom to bedetected. In other words, in the disposition example 706, thedistribution of the SEM signal amount at the line C-D, as shown in FIG.6( a), is likely not to be symmetrical at both sides of the position 315of the bottom 306, as shown in FIG. 6( d). However, in the dispositionexample 707, the distribution is substantially symmetrical, allowing aposition of a straight line near the true value to be detected.

In the disposition example 708, as seen in FIG. 7( b), multiple pyramidshaving different sizes (for example 710 and 711) are disposed. If thesize of the visual field changes due to changes in the imagingconditions, it is possible to select and observe pyramids with anappropriate size to secure an estimated accuracy of an observationaldirection.

The above-mentioned examples of dispositions can be applied to the anglespecimen 107 and the line and space type specimen 108 shown in FIG. 4(c) and FIG. 4( d), respectively. Further, in the angle specimen 107 andthe line and space type specimen 108, the two-dimensional (x- andy-direction) observational directions can be estimated by disposing acalibrator 712 (corresponding to 107 or 108) in such a manner that therolls of the shape of the specimen are bi-directional, as in the exampleof the disposition 709 shown in FIG. 7( b).

Tilt Angle Estimation Method

Definition of Relationship Among Wafer Surfaces, Calibrators andIncident Directions of Electron Beams

A method of estimation of the beam tilt angle by SEM observation usingthe beam tilt method will now be explained with reference to a concavepyramidal specimen used as a calibrator. FIG. 9( a) is a pattern diagramshowing the positional relationship among a wafer surface 203, a pyramid205 (a calibrator), and an incident direction 207 of an electron beam,when a concave pyramidal type specimen is observed. The wafer surface ismeans an area where no pyramid is formed on a surface of a specimenhaving a pyramid formed thereon. It is different from a semiconductorwafer used to form a semiconductor pattern actually being analyzed. Anexample of a method used to describe the wafer surface, pyramid andincident direction of an electron beam will now be explained. Theabsolute coordinate system 201 of the electron optical system is usedfor a standard of the positional relationship.

First, the slope of the wafer surface 203 shown in FIG. 9( a) can beexpressed with the unit surface normal vector 204 of the wafer surface.The unit surface normal vector 204 can be expressed as FIG. 9( b) byusing the rotation by and yx around the x-and y-axis of the unit surfacenormal vector 204 relative to the absolute coordinate system 201.

Then, the slope of the concave pyramid 205 shown in FIG. 9( a) can beexpressed with the unit orientation vector 206 of the center axis of theconcave pyramid.

The unit orientation vector 206 can be expressed shown in as FIG. 9( c2) by using the rotation φy and φx around the x- and y-axis of the unitorientation vector 206 relative to the absolute coordinate system 201.The central axis of the concave pyramid is a reference axis used touniquely define the orientation of a pyramid. For example, as shown inFIG. 9( c 1), it is the straight line 206 connecting the points with thesame distance from each edge P₀-P₁, P₀-P₂, P₀-P₃ and P₀-P₄ of a pyramid.The relative angle between the slope of the wafer surface and the slopeof the pyramid is known if the wafer surface 203 is a (100) crystalplane. In addition, with regard to the rotation of the concave pyramid205 (rotation of the crystal orientation) in FIG. 9( c 3), the angle ofthe projection vector of the wafer crystal orientation (100) vector onthe x- and y-plane with the x-axis can be expressed as φz.

Then, the incident direction 207 of an electron beam 207 shown in FIG.9( a) can be expressed with the unit orientation vector 208 in theincident direction of the electron beam 207. The unit orientation vector208 can be expressed as shown in FIG. 9( d) by using the rotation θy andθx around the x- and y-axis of the unit orientation vector relative tothe absolute coordinate system 201.

It is required to estimate a part or all of the parameters ψx, ψy, φx,φy, φz, θx and θy in order to estimate an incident direction (forexample, expressed by θx and θy) of an electron beam. In other words, itis a case where the equation to express the geometric relationshipbetween the observed amount (for example, a tilt angle of an edge of acalibrator) and an output variable (an incident direction of an electronbeam) can not be described without using parameters such as ψx, ψy, φx,φy, φz, θx and θy. However, at least the shape and tilt direction of acalibrator should be estimated or known for estimating a tilt angle.

The methods used to estimate the positional relationship of an incidentdirection of a wafer surface, pyramid and electron beam are not limitedto ones using the parameters ψx, ψy, φx, φy, φz, θx and θy. In addition,though an example using a pyramidal specimen as a calibrator is used inthe foregoing explanations, it is also possible to uniquely define atilt direction with calibrators other than pyramidal specimens.

Tilt Angle Estimation Method 1 (Basic Principle□ Single Image, an ImageOnly)

The method to estimate a beam tilt angle by processing a beam tilt imagewill now be explained. FIG. 6( a) shows a pattern diagram of a SEM imageof a concave pyramid observed from above. The changes in the beam tiltangle are reflected in the appearance of a pyramid. In other words, thelength and tilt angle of the segments 301 to 308 of the pyramid shown inFIG. 6( a) change depending on the beam tilt angle.

Definition of Output Parameters

Consequently, a parameter expressing a beam tilt angle can be estimatedby detecting a part or all of the segments 301 to 308 from an image andmeasuring the length and tilt angle. After that, a parameter to beestimated is called an “output parameter”. In this case, it is aparameter to express a beam tilt angle and is given by the parameters θxand θy. A output parameter group is expressed as an output parametervector U=(θx, θy)^(T).

Definition of Reference Parameters

The length or tilt angle of the pyramid also changes depending on thechanges of each parameter expressing the tilt angles of wafer surfacesor pyramids and image distortions, etc. In other words, each parameterexpressing the tilt angle of a wafer surface or pyramid and imagedistortions, etc should be selectively estimated as required in order toobtain an output parameter. These parameters that must be estimated inorder to obtain an output parameter are called a “reference parameter”hereinafter. In this case, they are parameters to express a tilt angleof a wafer surface or a pyramid and image distortion. For example, theyare given with the parameters ψx, ψy, φx, φy, φz. A reference parametergroup is expressed as a reference parameter vector V=(ψx, ψy, φx, φy,φz)^(T). A reference parameter is added or deleted as required. In otherwords, if the slope ψx and ψy of a wafer surface need not be estimatedfor estimating an output parameter, the slopes ψx and ψy are deletedfrom the reference parameter. Likewise, an image distortion needs to beestimated for estimating an output parameter, and so a parameterdescribing the image distortion is added to the reference parameter. Thecase where the slope ψx and ψy of a wafer surface need not be estimatedis a case where an estimation parameter is obtained by using only theinformation of the slope of the segments 301 to 304 on an observed imagein FIG. 6( a) as an observed amount. That is because a change in theslope of a wafer surface will affect the slope of the segments 305 to308 in FIG. 6( a), however, it does not affect the slope of the segments301 to 304.

The output parameter and reference parameter are collectively called an“estimation parameter”. An estimation parameter group is expressed as aestimation parameter vector T=(U^(T), V^(T))^(T).

3.2.3 A Method to Estimate an Estimation Parameter

FIG. 10 is a conceptual diagram of a method used to estimate anestimation parameter. The image 401 is processed by extracting segmentsfrom a SEM image or a reflective electron image of a calibrator that isimaged, or the SEM image or a reflective electron image. Combinations ofvaried values are assigned to each element of an estimation parametervector in order to obtain an estimation parameter vector T (405, FIG.10) at the time of imaging the calibrator 401, simulating whatcalibrator is observed in each combination. If the combination of variedvalues of each element is expressed as T(¹), T²), . . . , T(^(n−1)),T(^(n)), T(^(n−1)), by using the right upper superscripts, the shape ofa calibrator that is observed when the values of an estimation parametervector are T(^(n−1)), T(^(n)), T(^(n+1)) (406 to 408 respectively inFIG. 10) is given by 402 to 404, respectively, by use of a geometricequation.

The correlation between the simulated calibrator shape and thecalibrator shape in an image is calculated. Therefore, the estimationparameter vector T(n) (407) in the most similar simulation shape (403 inFIG. 10) corresponds to the estimation parameter vector 405 in an imagethat is taken. The methods to calculate the estimation parameter vectorwhere a taken image and simulation image correlate with each other atmaximum by changing the estimation parameter vector include, asdescribed above, a method to sequentially assign values and a method toanalytically obtain a solution by the least-square method, etc. Inaddition, the methods for a correlated calculation include a methodbased on the degree of similarity of the length or tilt angle of a partof or all of the segments 301 to 308 in a taken image and simulationimage as shown in FIG. 6( a), or the coordinate values of a part of orall of the vertexes P₀ to P₄ shown in FIG. 6( a).

In the explanation mentioned above, the output parameter to be finallyestimated is the beam tilt angle. The case where a stage tilt angle andbody tube tilt angle are estimated from the stage tilt method and bodytube tilt method is also the same. In other words, it is possible tocalculate an estimation parameter vector where a taken image andsimulation image match each other at maximum if a geometric shape changeon an image of a calibrator to any given estimation parameter can besimulated. For example, in the stage tilt method, an output parameterserving as an observational direction (an incident direction of anelectron beam to a specimen) may be a tilt angle of a calibrator becausethe calibrator mounted on the stage tilts while the stage tilts. A tiltangle of a calibrator is obtained by determining an estimation parametervector T, as shown in FIG. 10, in the same manner as the estimation of abeam tilt angle. In this case, estimation parameter vector T includes atilt angle of a calibrator as an output parameter. If an observationaldirection where a semiconductor wafer surface is perpendicularlyobserved is defined as a stage tilt angle 0°, when a tilt angle of apyramid relative to the observational direction where a semiconductorwafer surface is perpendicularly observed is φ₀, and a tilt angle of apyramid relative to the observational direction in any given tilt angleis φ, the stage tilt angle is given by φ-φ₀.

In addition, “a sequence to estimate a beam tilt angle using two images,a top-down image and a beam tilt image”, as mentioned below, includes astep to estimate a tilt angle of a calibrator by using a top-down image.Also, in the step, a tilt angle of a calibrator is estimated as anoutput parameter.

Tilt Angle Estimation Method 2 (Variation: Two Images or in Combinationwith other Measuring Means)

Varied sequences are assumed for the methods used to estimate theestimation parameters. They include, other than a method to estimate allestimation parameters at one time from a piece of observed image, (1) asequence to partially estimate an estimation parameter by dividing them,(2) a sequence to use multiple pieces of observed images, (3) a sequenceto estimate a part or all of estimation parameters by means other thanimages and to use the results of estimations or a sequence to combine(1) to (3). For example,

(a) a sequence to estimate a beam tilt angle using a top-down image anda beam tilt image, and

(b) a sequence to measure the slope of a wafer surface and the slope ofa calibrator by a measuring means other than image processing and toestimate a beam tilt angle by using the measurement results will now beexplained sequentially.

Methods to Estimate an Estimation Parameter-Variation 1

(a) A Sequence to Estimate a Beam Tilt Angle Using Two Images, aTop-Down Image and a Beam Tilt Image

In some cases, it is difficult to uniquely obtain an estimationparameter from a piece of beam tilt image. For example, if a tiltedcalibrator is observed on an image, it is sometimes difficult todetermine whether it is caused by the tilt angle 209 of a calibrator ora slope 210 of an incident direction of an electron beam because ofproblems concerning image quality or resolution, or insufficientinformation obtained from an image compared to the number of estimationparameters.

FIG. 11 shows a sequence used to estimate an estimation parameter byusing two pieces of image.

In this sequence, an image (called a “beam tilt image”) of a calibratorproduced by setting a tilt angle in the direction to be observed and animage (referred to as “top-down image” hereinafter) where a calibratoris observed with a beam tilt angle set to 0° are obtained, an actualtilt angle in a top-down image taken with a tilt angle set to 0° isassumed to be 0°, a tilt angle of a calibrator is estimatedindependently by using a top-down image, and a beam tilt angle isestimated independently by using a beam tilt image.

First, a top-down image is taken (step 501), a tilt angle and a rotationangle of a calibrator, and, if required, other reference parameters arecalculated by using the top-down image (step 502). In the step 502, atilt angle of a calibrator is calculated with a beam tilt angle that isknown (0°). Therefore, the uniqueness of a solution is higher than thecase where all estimation parameters are calculated at one time and thecalculation is easier. Then, a beam tilt image is taken after setting abeam tilt image in a direction to be observed (step 503). The true valueof the beam tilt angle and, if required, other reference parameters areestimated (step 504). In the step 504, a tilt angle is calculated with atilt angle of a calibrator that is known (a value calculated in the step502 is used) making the calculation easier than the step 502. There areseveral variations of order estimatable in which a specific estimationparameter is calculated. As shown above, the number of estimationparameters is reduced by dividing the parameters to be estimated,raising the uniqueness of solutions.

The calculations are performed here with a beam tilt angle 0° in atop-down image. However, in some cases, the actual beam tilt angle isdeviated from 0° in a top-down image taken with a beam tilt angle 0°. Inthose cases, the size of the deviation may affect the estimationaccuracy of a beam tilt angle after that. In an optical systemoriginally designed to obtain a top-down image, a higher settingaccuracy is requested in a beam tilt angle near the top-down. Therefore,estimating an actual beam tilt angle which is 0° in a top-down image,estimating a tilt angle by using a tilt angle of a calibrator estimatedfrom the top-down image as an input value can reduce the error whenestimating a tilt angle more than using a tilt angle of a calibratorestimated from an observed image with a tilt angle set to an angletilted over 0° as an input value.

In the stereo view detailed below, when a three-dimensional space isreconstructed by using the tilt images I_(L), I_(R) obtainedrespectively at the beam tilt angle set values θ_(L), θ_(R), it isexpected that the deviation Δθ° from the beam tilt angle 0° in thetop-down image little affects the three-dimensional space reconstructionerror. That is because, if a tilt angle is estimated in the tilt imagesI_(L), I_(R) respectively by using a tilt angle of a calibrator obtainedfrom a top-down image by the sequence as an input value (the estimatedvalues are Est[θL], Est[θR] respectively), the deviation of Est[θL],Est[θR] from the true value is almost the same as Δθ°. Therefore, therelative angle error of Est[θL], Est[θR] is small and the error in thethree-dimensional reconstructed shape by stereo view is also small.

The calculations after that are performed with a beam tilt angle in atop-down image set to 0°. In the same manner, an estimation parametercan be obtained by assigning an approximate value the parameter similarwith a parameter supposed to have no large error which is known. Themethods include, for example, a method in which the slope of a wafersurface is set to 0° approximately, etc.

Methods to Estimate an Estimation Parameter-Variation 2

(b) A Sequence to Measure the Slope of Wafer Surface and a Calibrator byMeans Other than Image Processing and to Estimate a Beam Tilt Angle byUsing the Measurement Results

Methods to measure an estimation parameter or the relationship betweenestimation parameters (relative values, etc) include a method whichemploys image processing and a distance measuring method by using an SPMor objective lens focused focal point, etc. An example of the method toestimate a beam tilt angle by using image processing in a selective orintegrated manner or other measuring means will now be explained.

FIG. 12 shows a beam tilt angle estimation method using an SPM,objective lens focused focal point and image processing. First, aspecimen is mounted on the SPM before a SEM observation. The specimensurface in area, including the wafer surface 602 and calibrator 604, ismeasured. The distance data 606 of the specimen surface obtained isdescribed based on the coordinate system 601 set in the SPM. Angles madeby planes constituting the specimen surface are obtained by multi-planeapproximating the distance data 606 by local plane assignment.Therefore, the relative angle between the tilt direction 603 of thewafer surface and the tilt direction 605 of the calibrator can becalculated.

Then, the specimen is mounted on the SEM and the slope direction 608 ofthe wafer surface 602 is estimated based on the distance measurementresult by the objective lens focused focal point. The distancemeasurement by the objective lens focused focal point is a method usedto estimate the distance to an objective to be observed when an actualobjective is observed by calculating the relationship between thecontrol current of the objective lens and the distance to the objectivewhen it is focused on in advance. A slope direction 608 is estimatedwith a plane normal line by assigning the planes, etc to the distancedata based on the objective lens focused focal point. The slopedirection 608 of the wafer surface 602 obtained is described based onthe coordinate system 607 set in a SEM.

The relative angle between the slope direction 608 of the wafer surfacein the coordinate system 607 and the slope direction 609 of a calibratoris equivalent to the relative angle between the slope direction 603 of awafer surface in the coordinate system 601 already measured and theslope direction 605 of a calibrator. The slope direction 609 in thecoordinate system 607 can be calculated by using the slope direction 608of the wafer surface and the relative angle. Assuming that the slopedirection of a calibrator serving as one of the reference parameters hasalready been calculated, the beam tilt angle 612 to the coordinatesystem 607 serving as an output parameter is calculated by imageprocessing. The accuracy to estimate the depth by the focused focalpoint of an objective lens is about several parts of the focal depth.Therefore, when the slope of a wafer surface is measured, themeasurement accuracy must be secured by calculating the slope based onlarge distance data (e.g., end to end of a specimen on a wafer).

Thus, in some methods, all estimation parameters are not measured byimage processing, but are measured by using measuring means such as anSPM or focused focal point of an objective lens in a selective orintegrated manner considering the measurement accuracy and usability ofeach measuring means. There are varied variations considered other thanthe combinations described above (e.g., all data other than the relativeangle between the slope direction 608 of a wafer surface and the slopedirection 609 of a calibrator is estimated by image processing).Further, it is not required to measure the positional relationshipbetween a wafer surface and a calibrator every time an estimationparameter is obtained because it is invariable once measured.

Imaging Sequence (Including Imaging of Objective Image)—Including aMethod to Correct an Estimated Value Accompanying Changes in ImagingConditions

The imaging sequence of a calibrator including imaging of asemiconductor pattern (refereed to as an “objective pattern”hereinafter) in any given coordinate system to be observed on asemiconductor wafer with a SEM will now be explained. The imagingsequences for a calibrator and objective pattern, including

(a) a sequence to image an objective pattern after imaging a calibrator(FIG. 13( a)),

(b) a sequence to image a calibrator after imaging an objective pattern(FIG. 13( b)) and

(c) a sequence to image a calibrator off-line (FIG. 14) will now beexplained sequentially. In the sequences (a) and (b), an objectivepattern and a calibrator are imaged alternately. Therefore, a calibratorspecimen 1319 is mounted on the position 705 in FIG. 7( a) whereattachments and removals of a semiconductor wafer are not required. Inthe sequence (c), a calibrator specimen 1319 is mounted on the position704 or 705 in FIG. 7( a).

Imaging Sequence-Variation 1

(a) A Sequence to Image an Objective Pattern After Imaging a Calibrator(FIG. 13( a))

This is a sequence, in which a calibrator is first imaged with any giventilt angle set, where the setting is not changed basically (it may bechanged when a tilt angle is calibrated), then, an objective pattern isimaged. When there are hysteresis characteristics between a tilt angleset value and an actual tilt angle, even if any given tilt angle setvalue is changed to a different set value temporarily and it is returnedto the original tilt angle set value, the actual tilt angles in theformer and latter tilt angle set value may not be the same. Thissequence avoids the risk that a tilt angle estimated by a calibrator cannot be used in an analysis of an observed image of an objective pattern.

This sequence will now be explained with reference to FIG. 13( a).First, a semiconductor wafer is mounted on a SEM (step 800). Then, animaging point is set on a specimen on which a calibrator is formed (step801). A tilt angle is changed into any give set value (step 802). Thereis no problem if the order of the steps 801 and 802 is reversed. Aftersetting a tilt angle, a calibrator is imaged in the step 804. Ifrequired, the imaging conditions are changed in the step 803 beforeimaging the calibrator. If required, multiple calibrators are imaged inmultiple imaging conditions with the loop 810.

In the loop 810, multiple calibrators are imaged while changing theimaging conditions. In the loop 810, a calibrator is imaged by thecombination of imaging conditions required for estimating a tilt anglein an objective pattern image group to be imaged in the loop 811 later.Further, it is expected that the reliability of estimated values isimproved by estimating a tilt angle from multiple calibrator images.Therefore, if a tilt angle in the objective pattern image group can beestimated by the observed images of a calibrator obtained by one imagingcondition, the loop 801 is not used. The changes in the imagingconditions include stage movements, image shifts (movements of visualfield due to a changed irradiation position of an electron beam),magnification changes, focus changes, astigmatic corrections, etc. Inimaging, an imaging point may be changed, or the image may be enlargedor reduced. At that moment, the focus may be changed or astigmaticcorrections are performed to obtain good images. Further, if required,an estimated value of a tilt angle is calculated from a calibrator imageobtained in the step 804 and the tilt angle is calibrated in the step805 in order to eliminate the deviations between the estimated value ofthe tilt angle and the set value of the tilt angle.

An observed image can be obtained in the set observational direction inthe step 808 by performing the calibration. The calibration method fortilt angles will be detailed later (FIG. 15). If required, thecalibrator imaging (step 804), tilt angle estimation, tilt anglecalibration (step 805) are repeated in the loop 810 until an estimatedvalue of a tilt angle does not deviate from a set value of a tilt angle.After correcting a tilt angle (step 805), a calibrator is imaged again(step 804). Confirm that there is no deviation between an estimatedvalue and a set value of a tilt angle. If the step 805 is skipped toimage an objective pattern, the set value of a tilt angle is the samebetween the step 804 and the step 808. If the calibrator is not imagedagain, the set value of the tilt angle is different between the steps804 and 808. However, if a tilt angle on imaging an objective patterncan be estimated within an allowable error required for the analysis ofthe object pattern later, there is no problem.

Then, the imaging position is moved to the semiconductor wafer (step806). An objective pattern is imaged in the step 808. If required, theimaging conditions are changed before imaging an objective pattern inthe step 807. If required, multiple objective patterns are imaged by theloop 811.

Imaging Sequence-Variation 2

(b) A Sequence to Image a Calibrator after Imaging an Objective Pattern(FIG. 13( b))

In this sequence, any given tilt angle is set, an objective pattern isimaged in any given imaging condition, and then, a calibrator is imagedwith the tilt angle setting unchanged to estimate a tilt angle. Thissequence has an advantage in that the imaging conditions in observingthe objective pattern can be determined based on the imaging conditionsin observing the objective pattern by observing a calibrator afterimaging an objective pattern. In other words, the tilt angle may bechanged if the imaging conditions are changed. Therefore, a tilt anglewhen imaging an objective pattern can be estimated more correctly byimaging a calibrator in the imaging conditions similar to theobservations of the objective pattern.

This sequence will be explained with reference to FIG. 13( b). First, asemiconductor wafer is mounted on a SEM (step 813). Then, an imaging isset on the semiconductor wafer on which an objective pattern is formed(step 814) and the tilt angle is changed into any given set value (step815). There is no problem if the order of the steps 814 and 815 isreversed. After setting a tilt angle, an objective pattern is imaged inthe step 816. If required, the imaging conditions are changed in thestep 815 before imaging the objective pattern. At the same time, ifrequired, multiple objective patterns are imaged in multiple imagingconditions with the loop 821.

Then, the imaging position is moved on a specimen on which a calibratoris formed (step 817). The calibrator is imaged in the step 819. Beforeimaging the calibrator, if required, the imaging conditions are changedin the step 818. At the same time, in the loop 822, images of thecalibrator are taken in multiple conditions required for estimating atilt angle when imaging an objective pattern in the step 816.

In FIG. 13( a) and FIG. 13( b), the imaging condition change (step 803,807, 814, 817) and imaging position change (step 801, 806, 814, 817) areseparated. The imaging condition change includes the stage movement andimage shift. This means a stage movement or an image shift betweencalibrators or objective patterns. Meanwhile, the imaging positionchange means a stage movement between a calibrator and an objectivepattern. If the tilt angle when setting any given tilt angle shown inFIG. 11 is estimated by the combination of the tilt image and top-downimage in setting the tilt angle of a calibrator, the imaging of anobject pattern (step 808 and 816) with the tilt angle set to 0°(top-down) is skipped.

Imaging Sequence-Variation 3

(c) A Sequence to Image a Calibrator Off-Line (FIG. 14)

In this sequence, a calibrator is imaged by the combination of a tiltangle and imaging conditions to be required for estimating a tilt anglebefore inputting a semiconductor wafer or imaging an objective pattern,and the image group and the tilt angle estimated by the image group issaved in a data base with the imaging conditions. The tilt angle in anobjective pattern imaged in any given imaging conditions later can beestimated by the data base.

This sequence will be explained with reference to FIG. 13( c).Calibrators at multiple areas are imaged by the combination of a tiltangle to be imaged and imaging conditions (The tilt angle change,imaging condition change and imaging are performed in the steps 901,902, and 903, respectively). The image of a calibrator is saved in adata base 905. If required, an estimated value of a tilt angle iscalculated in any given tilt angle set value or imaging conditions inthe image processing part 902 by using an image group saved in the database. The estimated value is saved in the data base 905 in the form of alibrary. A tilt angle in an objective pattern imaged in any givenimaging conditions later can be estimated from the data base by usingthe data base. Even if an image of a calibrator imaged in the sameconditions as the tilt angle set value or imaging conditions in imagingan objective pattern is not saved in the data base, an estimated valueof a tilt angle in imaging the objective pattern can be calculated by aninterpolation, etc from an estimated value of a tilt angle imaged byother conditions in the data base.

In imaging a calibrator in the three imaging sequences shown in FIG. 13(a) and FIG. 13( b) and FIG. 14, the deviations in the estimated valuesof the observational direction caused by the individual difference andimage noises can be reduced by observing single calibrators or multipledifferent calibrators disposed sequentially, estimating an observationaldirection in each image and using an average or median in theobservational direction.

Estimated Value Correction Accompanying Imaging Condition Change

The actual tilt angle may be different between the objective patternimaging and the calibrator imaging even if the set tilt angle is notchanged between them. Therefore, the tilt angle can be estimated byestimating the variation of the tilt angle accompanying the imagingcondition changes, and adding the variation to the estimated tilt valuein imaging a calibrator.

The method to calculate the variation of the tilt angle accompanying thechanges in the imaging conditions will now be explained.

The variation of a tilt angle due to a stage movement, image shift ormagnification change among the changes in the imaging conditions can beestimated from the distribution of the tilt angle in the semiconductorwafer plane (e.g., estimated from a tilt angel of a calibrator mountedat the position 704 in FIG. 7) or distribution of tilt angles in thevisual field (e.g., estimated from a tilt angle in a calibrator 706 inFIG. 7). Especially, with regard to the image shift, the changes in theincident direction of an electron beam due to the shift of an electronbeam can be estimated by a geometric calculation of an optical system.In addition, the variation in the tilt angle due to changes in the focusamong the changes in the imaging conditions can be estimated byexperimentally or analytically calculating the relationship between thecontrol current value of an objective lens and the tilt angle. In otherwords, the focus is changed by changing the control current of anobjective lens. Therefore, a tilt angle can be estimated by using acontrol current of an objective lens in observing an objective patternif the relationship between the control current and tilt angle of theobjective lens is obtained.

Use of Estimated Results (Tilt Angle Calibration, 3D Reconstruction, GUIDisplay, etc)

The uses of an estimated tilt angle in observations or measurements ofan observed image will now be explained.

Calibration of Tilt Angles

A tilt angle can be calibrated with an estimated value of a tilt anglewith a calibrator and a set value, and the actual tilt of a tilt anglecan be matched with each other (corresponding to the step 808 in FIG.13).

FIG. 15 shows a sequence used to calibrate a tilt angle. First, anygiven tilt angle to be observed is set. Then, an image of a calibratoris obtained (step 1001, corresponding to the step 804 in FIG. 13( a)).An estimated tilt angle value is calculated from the image of acalibrator(step 1002, corresponding to the determination of anestimation parameter T shown in FIG. 10 mentioned above). The differenceΔθ between the estimated value and set value of the tilt angle iscalculated. If the absolute value of Δθ is larger than the threshold setin advance (condition 1004), the deflection of an electron beam ischanged in the beam tilt method or the slope of the stage is changed inthe stage tilt method in such a manner the absolute value of the Δθ isreduced (step 1005). After that, the calibrator is observed and a tiltangle is estimated by the loop 1006. The re-calculated Δθ is judged toconfirm whether to meet the condition 1004 is met. This process isrepeated until the condition 1004 is met to make the estimated value ofthe tilt angle convert to the set value.

CD Measurement or Shape Index Calculation

By the measurement of a critical dimension or the other dimensionscorresponding to any part of semiconductor patterns in a top-down viewor tilt view, as disclosed in the Japanese Published Unexamined PatentApplication No. 2000-022617, it is possible to estimate thesemiconductor pattern 3D shape or calculate indices which have highcorrelation with the pattern 3D shape. The critical dimension and theother dimensions are called “various CDs” in all hereinafter.

However, a setting error of the tilt angle leads to the measurementerror of various CDs.

The present calibration method makes it possible to estimate a reliablemeasurement value of various CDs by following either of two disposals.

-   (1) To correct measured various CD values by using estimated tilt    angle-   (2) To measure various CDs in the adjusted image with a desired tilt    angle (setting error of tilt angle is corrected by the calibration)

FIG. 19 shows, as an example of the disposals (1) above-mentioned, amethod to calculate correct side wall length W₁ (observation length intop-down view) in the case of estimation of side wall angle ω.

If the height of step H is known, the side wall angle ω is given bytan⁻¹ (H/W₁).

However, if the observation direction includes an angle error Δθ, theside wall length also includes a length error ΔW and side wall lengthbecomes W₂=W₁ΔW.

The present calibration method can estimate the angle error Δθ andcalculate the correct side wall length W₁=W₂−ΔW=W₂−H tan(Δθ).

The disposal can be also applicable similarly to the case of the otherpart of semiconductor pattern and tilt angle variations.

3D Reconstruction

In a 3D profile reconstruction by using an observed image of anobjective pattern from multiple directions, a higher reconstructionaccuracy is obtained by (1) using a tilt angle estimated with acalibrator as an input value of an observational direction or (2) usingan observed image and a set value of a tilt angle of an objectivepattern imaged in the observational direction equivalent to the setvalue by calibrating the tilt angle and a set value of a tilt angle asan input value.

FIG. 16 shows, as an example of an algorithm used to reconfigure a 3Dprofile, a method to calculate the step (height) between the two pointsP₁-P₂ in the section 1101 of a measuring object. For example, theabsolute coordinate system 201 in a SEM is used as a standard for thedirection to measure the step. The height in the z-axis direction of theabsolute coordinate system 201 is measured as a step. In FIG. 16( a),L₁, and L₂ are the distances between the two points P₁-P₂ on an imageobserved from above with beam tilt angles θ₁ and θ₂, respectively. Thestep H between the two points P₁-P₂ is given by H={(L1−L2)·cos θ₁·cosθ₂}/sin(θ₁−θ₂) by using L₁, L₂, θ₁ and θ₂ through a geometriccalculation.

In the same way, in FIG. 16( b), M₁ and M₂ are the distances between thetwo points P₁-P₂ observed from above with a stage tilt angles θ₁ and θ₂,respectively in a image. The step H between the two points P₁-P₂ isgiven by H=(M₁·cos θ₂−M₂·cos θ₁)/sin(θ₁-θ₂) by using M₁, M₂, θ₁, θ₂through a geometric calculation. The input errors of the tilt angles θ₁and θ₂ affect the 3D profile reconstruction both in the beam tiltobservation and stage tilt observation. Therefore, the reconstructionaccuracy can be improved by correctly estimating and inputting theobservational direction in an observed image.

FIG. 16( a) and FIG. 16( b) show an example of calculations of the stepH between the points P₁-P₂. The irregularities on the surface of anobject pattern can be measured more precisely irrespective of theoutside or inside of the points P₁-P₂. FIG. 16( c) shows a method usedto measure the points P₁-P₇ on the surface of an objective pattern byusing abeam tilt image. For example, the step H₅-₆ between the pointsP₅-P₆ is given by H₅₋₆={(L₁, ₅₋₆-L₂, ₅₋₆)·cos θ₁·cos θ₂}/sin(θ₁-θ₂) inthe same manner as the case shown in FIG. 16( a) by using the distancesL₁, ₅₋₆ and L₂, ₅₋₆ between the two points observed from above with beamtilt angles θ₁ and θ₂, respectively. The quality of a semiconductorpattern and the semiconductor process variations can be checked andcontrolled by measuring the height, line width and side wall slope angleof the objective pattern, and subtle pattern shape such as roundedcorners from the measurement results obtained.

In FIG. 16( d), the points P₁-P₇ (indicated with the white circles inFIG. 16( d)) on the objective pattern surface 1101 obtained from thedesign data, etc and the measurement points P′₁-P′₇ of the height of thepoint P₁-P₇ measured by each tilt image (indicated with the blackcircles in FIG. 16( d), 1102 is the multi-point approximate shape of theblack circles connected to one another on the objective pattern surface)are overlapped. In this example, it is clear that the step is lower thanthe design value in the shape (the shape of the measurement data) of theobjective pattern completed and the corner near the point P1 is rounded.Thus, the objective pattern status can be evaluated by displaying theshape of the design data and measurement results set out or overlapped,or displaying the shapes, such as the height and roundness of thecorners, etc digitized. In addition, for example, as shown in FIG. 17(a), the 3D shape measurement results 1205 can be obtained bytwo-dimensionally measuring the multi-points on an objective patternsurface in the direction of the x- and y-axis of the absolute coordinatesystem 201.

GUI Display

An estimated value of the actual tilt angle can be displayed on a GUI ora beam tilt image can be converted to a stage tilt image for display byusing an estimated value of a tilt angle by a calibrator.

FIG. 17( a) shows an example of a GUI (Graphic User Interface) todisplay the estimation results of an observed image and tilt angle. Theimage displays 1201 and 1202 are the beam tilt images with the set valueof the observational direction 5° and 10° respectively (referred to as“image 01” and “image 02” respectively hereinafter). The image displays1203 and 1204 are the stage tilt images estimated and displayed at theestimated value of the tilt angle of the set value or the tilt angle bythe calibration from the beam tilt images 1201 and 1202. A stage tiltimage can be estimated from a beam tilt image and an observationaldirection to be displayed even if a stage tilt image is not observedactually.

It is difficult to interpret a beam tilt image obtained by deflecting anelectron beam intuitively because it is geometrically different from anoblique perspective image seen with the naked eyes of a human.Therefore, converting an image into a stage tilt image which iscomparable to an observation by a human obtained by tilting a stage iseffective for analyses with the human eye. Further, in the case where astage tilt image is observed, it is possible to reversely convert itinto a beam tilt image to be displayed. Still further, in the imagedisplays 1201-1204, an image with the distortion corrected can bedisplayed by using the results of the image distortion estimation, anexample of which is shown in FIG. 7( c).

1205 denotes a 3D display of a 3D profile estimated from an observedimage. The 3D profile can be displayed in any given observationaldirection.

1206 denotes vectors three-dimensionally expressing the set value of theobservational direction in the image 01 and 02, and the estimated valuesof the observational direction using a calibrator. The components of thevector (the set value and estimated value of the tilt angle in the x-and y-direction) are displayed at 1207. Further, the 3D profile and theobservational direction can be two-dimensionally displayed as shown inFIG. 17( b) at 1208 and 1209, respectively. In FIG. 17( b), 1208 and1209 denote the estimated 3D profile and the observational directionprojected on the x-z plane as an example, respectively. (1)-(4) in 1206,1207 and 1209 correspond to one another. Further, the errors between theset value and estimated value in an observational direction, an observedimage of a calibrator and a slope direction of a calibrator and wafercan be also displayed.

As shown above, any give combination of a beam tilt image of anobjective pattern (1201, 1202), a stage tilt image(1203, 1204), a 3Dprofile of an estimated objective pattern (1205) a 3D display of anobservational direction (1206), a two-dimensional display of anobservational direction (1209 or 1102 in FIG. 16( d)), a digitizeddisplay of an observational direction (1207) and a design shape of anobjective pattern obtained from design data, etc (1101 in FIG. 16( d))can be displayed on the same GUI.

In the above-described examples of the tilts observed mainly in the beamtilt method and the pyramidal specimens used as calibrators, methods toarrange the calibrators, methods to detect the geometric deformations inthe observed images, methods to estimate the observational directions,sequences to estimate the taken images and observational directions havebeen explained. However, the present invention is not limited to thoseexamples. The present invention can be applied to other methods toobserve tilts and calibrators in the same manner.

In accordance with the present invention, correct observations andmeasurements can be performed by using the tilt observation images ofthe objective patterns and correct estimated values of observationaldirections. Further, even in the SEM devices with hysteresischaracteristics (when the actual tilt angle deviates every time a tiltangle is set even if the same value is set to the tilt angle) or with abad repeatability, a tilt angle can be observed and measured in a moreaccurate and repeatable fashion by estimating and calibrating the tiltangle. Further, even when the same specimen is observed with differentSEM devices, the difference in the tilt angles by the different SEMdevices is estimated, the difference between devices due to thedifference between the tilt angles is corrected, and the samemeasurement results can be obtained. The measurements include, forexample, measurements of dimensions, such as line widths and contacthole diameters, and the above-mentioned 3D profile measurements. Thevariations of the semiconductor processes can be effectively detected bythe measurement values with a high accuracy. Further, in the case wherean objective pattern with a surface tilt angle more than 90°, called an“inverse tapered shape”, is measured (there is an area that can not bemeasured when observed from just above), the tilt observation isespecially effective. The observation and measurement methods inaccordance with the present invention are very important for theanalyses with a high degree of accuracy.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiment is therefore to be considered in all respects as illustrativeand not restrictive, the scope of the invention being indicated by theappended claims rather than by the foregoing description and all changeswhich come within the meaning and range of equivalency of the claims aretherefore intended to be embraced therein.

1. A method of observing a specimen, comprising the steps of:irradiating and scanning a convergent electron beam, from an obliquedirection, on surface of a calibration substrate on which a pattern witha known shape is formed, and thereby obtaining a beam tilt SEM image ofthe pattern formed on the calibration substrate; calculating an angle ofthe oblique direction of the electron beam irradiated on the surface ofthe calibration substrate by use of the information about a distortionof the beam tilt SEM image of the pattern estimated from a geometricdeformation of the beam tilt SEM image of the pattern; adjusting theangle of the oblique direction of the electron beam to a desired angleby using information of the calculated angle; irradiating the electronbeam from the adjusted desired angle on a specimen substrate on which apattern is formed, and thereby obtaining a beam tilt SEM image of thepattern formed on the specimen substrate; and processing the beam tiltSEM image of the pattern formed on the specimen substrate by use of theinformation of the desired angle, and thereby, in combination withinformation of another SEM image of the pattern formed on the specimensubstrate, obtaining a 3D image of the pattern formed on the specimensubstrate or a shape of a cross section of the pattern formed on thespecimen substrate.
 2. A device for observing a specimen comprising: atable which carries a specimen and which is set at a desired tilt angle;a SEM image obtainer which obtains a SEM image including a beam tilt SEMimage by irradiating and scanning an electron beam onto a specimencarried on the table from a desired incident angle, and which detects asecondary electron or reflective electron generated from the specimen bythe irradiation of the electron beam; an image processing unit whichprocesses an image obtained by the SEM image obtainer and displays aprocessed image on a screen; a controller which controls the SEM imageobtainer; and a calculator which calculates an incident angle of theelectron beam irradiated on a calibration substrate, on which a patternwith a known shape is formed, from a beam tilt SEM image of thesubstrate on which the pattern with the known shape is formed by usinginformation about the shape of the pattern with the known shape and theinformation about a distortion of the beam tilt SEM image of the patternwith the known shape estimated from a geometric deformation of the beamtilt SEM image of the pattern with the known shape; wherein the imageprocessing unit processes a SEM image of a pattern formed on a specimensubstrate and, in combination with information of another SEM image ofthe specimen substrate obtained by the SEM image obtainer, obtains a 3Dimage of the pattern formed on the specimen substrate or a shape of across section of the pattern formed on the specimen substrate based onthe information of the incident angle of the electron beam calculated bythe calculator, and displays the obtained information of the 3D shape orthe shape of a cross section of the specimen on a screen.
 3. A methodfor observing a specimen according to claim 1, wherein the pattern withthe known shape formed on the calibration substrate has a crystal planeexposed by etching materials.
 4. A method for observing a specimenaccording to claim 1, wherein the pattern with the known shape is formedby exposing a plane (III) exposed by etching silicon or a plane in adirection equivalent to the plane (III).
 5. A device for observing aspecimen according to claim 2, wherein the pattern with the known shapeformed on the calibration substrate has a crystal plane exposed byetching materials.
 6. A device for observing a specimen according toclaim 2, wherein the pattern with the known shape is formed by exposinga plane (III) exposed by etching silicon or a plane in directionequivalent to the plane (III).
 7. A method of observing a specimen,comprising: irradiating and scanning a convergent electron beam, from anoblique direction, on a surface of a calibration substrate on which apattern with a known shape is formed, and thereby obtaining a beam tiltSEM image of the pattern formed on the calibration substrate;irradiating and scanning the convergent electron beam on the surface ofthe calibration substrate and obtaining a top-down SEM image of thepattern; calculating an angle of the oblique direction of the electronbeam irradiated on the surface of the calibration substrate by use ofthe information about the shapes of the pattern which the known shapeand the information about a distortion of the beam tilt SEM image of thepattern estimated by using the top-down SEM image of the pattern;adjusting the angle of the oblique direction of the electron beam to adesired angle by using information of the calculated angle; irradiatingthe electron beam from the adjusted desired angle on a specimensubstrate on which a pattern is formed, and thereby obtaining a beamtilt SEM image of the pattern formed on the specimen substrate; andprocessing the beam tilt SEM image of the pattern formed on the specimensubstrate by use of the information of the desired angle, and thereby,in combination with information of another SEM image of the patternformed on the specimen substrate, obtaining a 3D image of the patternformed on the specimen substrate or a shape of a cross section of thepattern formed on the substrate.
 8. A method for observing a specimenaccording to claim 7, wherein the pattern with the known shape formed onthe calibration substrate has a crystal plane exposed by etchingmaterials.
 9. A method for observing a specimen according to claim 7,wherein the pattern with the known shape is formed by exposing a plane(III) exposed by etching silicon or a plane in a direction equivalent tothe plane (III).
 10. A device for observing a specimen comprising: atable which carries a specimen and which is set at a desired tilt angle;a SEM image obtainer which obtains a top-down SEM image and a beam tiltSEM image by irradiating and scanning an electron beam onto a specimencarried on the table from a desired incident angle, and which detects asecondary electron or reflective electron generated from the specimen bythe irradiation of the electron beam; an image processing unit whichprocesses an image obtained by the SEM image obtainer and displays aprocessed image on a screen; a controller which controls the SEM imageobtainer; and a calculator which calculates an incident angle of theelectron beam irradiated on a calibration substrate, on which a patternwith a known shape is formed, from a beam tile SEM image of the patternby using information about the shape of the pattern and the informationabout a distortion of the beam tilt SEM image of the pattern estimatedby using the top-down SEM image of the pattern; wherein the imageprocessing unit processes a SEM image of a pattern formed on a specimensubstrate and, in combination with information of another SEM image ofthe pattern formed on the specimen substrate obtained by the SEM imageobtainer, obtains a 3D image of the pattern formed on the specimensubstrate or a shape of a cross section of the pattern formed on thesubstrate based on the information of the incident angle of the electronbeam calculated by the calculator; and displays the obtained informationof the 3D shape or the shape of a cross section of the specimen on ascreen.
 11. A device for observing a specimen according to claim 10,wherein the pattern with the known shape formed on the calibrationsubstrate has a crystal plane exposed by etching materials.
 12. A devicefor observing a specimen according to claim 10, wherein the pattern withthe known shape is formed by exposing a plane (III) exposed by etchingsilicon or a plane in a direction equivalent to the plane (III).